Does anyone know the spindle length I need to run my Hollowgram cranks on a Specialized OSBB?

Same. Specialized' OSBB = BB30

The only difference is marketing.

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Exp001

I get that, but the Hollowgram spindle comes in different lengths, and want to make sure the one that I have from my Supersix (104mm I believe) will work. Or maybe I'm confusing myself.

The Hollowgram road spindles are the same length. The MTB spindles are longer. However, there is an "SRM version" of the road spindle that has a slightly different amount of offset from the regular one, though it's the same overall length. I think the SS Evo uses this version of the spindle even with non-SRM cranks.